1. Field of Invention
Aspects of the present invention are directed to a heater, and more particularly to a resistive heater that may be mounted to a surface through which a fluid is conveyed.
2. Discussion of Related Art
In a number of different fluid processing and distribution systems, it can be important to control the temperature of the process fluids (liquids, gases, liquids and gases, slurries, etc.) being used. For example, in the semiconductor industry, cartridge type heaters are frequently inserted into substrates of a fluid stick and used to heat the process fluids that are delivered to a tool. These cartridge type heaters are generally elongated cylindrically shaped resistive-type heaters that are inserted into a cavity or well opening formed in the substrates that are used to convey the fluid to the process tool(s). Commonly owned U.S. patent application Ser. No. 10/650,102, filed Aug. 26, 2003, entitled MODULAR SUBSTRATE GAS PANEL HAVING MANIFOLD CONNECTIONS IN A COMMON PLANE, (hereinafter “the co-pending application”), which is herein incorporated by reference, illustrates a modular substrate gas panel in which the modular substrates have such a cavity formed therein.
As shown in FIG. 17 of the co-pending application, reproduced herein as FIG. 6, gas panel 200 includes a plurality of gas sticks A-L, in which each gas stick includes a pair of cavities 500 within which a cartridge-type heater (not shown) may be inserted. In use, each of the substrates forming a gas stick has similar cavities, such that when the substrates of the gas stick are aligned, the cartridge heater inserted in cavities or well openings 500 may span the length of the gas stick. The cartridge heaters operate by thermally transferring heat to the substrates, which are typically formed from a thermally conductive material, such as stainless steel.
As known to those skilled in the art, cartridge heater efficiency is largely determined by the tolerances and positions of the heater well openings (i.e., cavities) required for cartridge insertion. If a cartridge has too much clearance between the opening wall and the heater sheath (i.e., the outer surface of the cartridge heater), heat transfer rates are reduced, heater sheath temperature increases, and power demands increase. In the semiconductor industry, a major cause of cartridge heater failure is improper insertion into heater wells or improper well design. Cartridge heaters often fail by welding themselves into the well opening when heater sheath temperature exceeds the maximum safe temperature for that heater. This causes service nightmares, as heaters may need to be drilled out before they can be replaced, and also raises numerous potential safety issues.
Cartridge heaters operating properly create very high temperature gradients in the area of the heater wells, as the temperature at the heater surface is often 200% to 500% of the desired system temperature. This causes not only thermal stresses across the gas stick, but also presents a challenge in controlling the temperature in the desired zone, since the heater must be located physically separate from the target zone (i.e., the channel formed in the substrate in which the process fluid flows).
The failure rate of cartridge type heaters can be staggeringly high, and is generally determined more by the installation method than by the heater quality or system design. An improperly installed cartridge heater will lead to rapid heater failure, and in practice, improper installation often cannot be tested or seen until the heater has failed, when it is too late.